1. Field of the Invention
The present invention relates to an electrically-driven fan for forcibly cooling the CPU of a personal computer, or the like.
2. Description of the Related Art
In recent years, the higher performance of a personal computer (hereinbelow, termed “PC”) has been attained. In consequence, the clock frequency of a CPU heightens to increase dissipation power and to increase the quantity of heat generation. The CPU whose temperature has risen due to the heat generation becomes unstable in the operations of transistors, and undergoes the drawbacks of failing to operate normally, etc. It is therefore necessary to forcibly cool the CPU and to release generated heat. A heat sink, a heat pipe or the like is usually used for the forcible cooling of the CPU, and further, an electrically-driven fan is extensively used with or without the heat sink, the heat pipe or the like. Some of such electrically-driven fans employ commutatorless DC motors in each of which a cylindrical magnet is employed for a rotor, and they are extensively used for such reasons as being capable of small size and thin structure and being easy of control. A “thin-structured electrically-driven fan” has been disclosed as the electrically-driven fan employing the commutatorless DC motor. Incidentally, the “thin-structured electrically-driven fan” employs a ball bearing as the bearing of a rotary shaft, but it sometimes employs a fluid bearing instead of the ball bearing. The fluid bearing is a bearing of noncontact type wherein V-shaped recesses are formed along the inner peripheral wall of a bearing hole, and oil is accumulated in the recesses, whereby the rotary shaft is floated and supported by the oil. The fluid bearing is capable of high-speed rotation owing to a very low friction, and it exhibits low rotation noise, so that it has been extensively used in recent years (refer to, for example, JP-A-11-150908 and JP-A-2002-112497).
On the other hand, the PC has been reduced in size in recent years, and especially in a notebook type PC or the like, various electronic components including the CPU are installed at a high density within a housing. Accordingly, an electrically-driven fan for cooling the CPU of such a PC has various limitations placed on its mounting position in order to realize a space-saving arrangement, and it is sometimes mounted looking down in such a way, for example, that it is fixed to a ceiling inside the housing. A vertical direction is a direction indicated in FIG. 8.
The prior-art techniques, however, have had problems as stated below.
(1) Usually, in order to attain enhancement in a coolability with a small space, the radiating blades of the rotor have been enlarged, or the number of the blades has been increased, with the result that the weight of the rotor has increased. The thin-structured electrically-driven fan has been problematic in the case where the electrically-driven fan is mounted looking down, as follows: When the weight of the rotor is large, the rotor moves vertically downwards due to its weight, and the position of the rotary shaft shifts downwards relative to the inner peripheral wall of the bearing hole, so that the abnormal touch between the rotary shaft and the inner peripheral wall of the bearing hole occurs to shorten the lifetime of the electrically-driven fan.
(2) Especially the thin-structured electrically-driven fan employing the fluid bearing has been problematic, as follows: The increase of the weight of the rotor results in a state where the position of the rotary shaft shifts downwards relative to the inner peripheral wall of the bearing hole, namely, a so-called state where the rotor rises, and the rotation of the rotor is not stabilized to incur the drawbacks that vertical vibrations occur during the rotation, and that the abnormal touch between the rotary shaft and the inner peripheral wall of the bearing hole occurs to generate noise.
(3) Besides, there has been the problem that, when the weight of the rotor is large, the position of the rotary shaft shifts downwards relative to the inner peripheral wall of the bearing hole, so the abnormal touch between the rotary shaft and the inner peripheral wall of the bearing hole occurs to shorten the lifetime of the electrically-driven fan. A sole measure for coping with this problem has heretofore been that the radiating blades are made small to lighten the rotor, or that the mounting position of the cylindrical magnet on the rotor is altered to mount the cylindrical magnet on a position distant from a rotor position detector such as Hall element. The measure has been problematic in that the enhancement of the coolability cannot be expected, or that the detection precision of the rotor position detector lowers.